Technical Field:
The present disclosure relates to vibration attenuation for rotor hubs.
Description of Related Art:
Rotary-wing aircraft, such as helicopters and tiltrotors, have at least one rotor for providing lift and propulsion forces, and these rotors have at least two airfoil blades connected to a rotatable hub. The blades cause vibrations that are a function of the rotational speed of the rotor, and aircraft designers have difficulty accurately predicting the exact vibration modes that a particular rotor configuration will encounter. The vibrations can be transmitted through the rotor hub, through the rotor mast, through associated powertrain components, and into the airframe of the aircraft. The vibrations can reduce the life of affected components and cause undesirable vibration for passengers. Various types of vibration attenuation systems have been developed to reduce or eliminate these vibrations. The prior art includes both passive and active devices that are airframe-mounted, mounted at the interface between the airframe and the rotor system, and devices mounted in the rotor system.
Conventionally, active control of helicopter vibration has been by one of three methods; installing force generating actuators onto the airframe, or installing actuation devices between the main rotor pylon assembly and the airframe, or installing actuation devices in the main rotor system. Active vibration control systems, for rotorcraft applications, are closed loop control systems designed to suppress vibrations at harmonics of the main rotor frequency. Typically, a rotorcraft active vibration control system is comprised of vibration sensors; a control computer to process the vibration measurements and output control commands to an actuation device; and actuation devices to produce force inputs to the rotorcraft airframe, or at the pylon-airframe interface, or actuation inputs to the rotor hub, or actuation inputs to the rotor blades. Vibration reduction is achieved by superposition of the vibrations created by the active system's actuation forces, and the vibrations caused by the rotorcraft main rotor system.
Active vibration control systems have been proposed that are mounted to the main rotor hub and apply control forces to the main rotor hub. These types of systems propose two or more pairs of masses revolving about and concentric with the center of the main rotor mast to create a controllable force to cancel the rotor generated forces.
Although great strides have been made in the art of vibration attenuators for rotorcraft, there is an on-going need for vibration suppression devices and methods that are more effective, require less weight, require less power, and have less severe failure modes.